The present invention relates to a semiconductor thin film crystallization device and a semiconductor thin film crystallization method, for crystallizing semiconductor thin film by using laser.
In recent years, thin film transistors (hereinafter, referred to as TFTs) using polysilicon film have been drawing attention among TFTs under development. In particular, LCDs (Liquid Crystal Displays) or EL (Electro-Luminescence) Displays employ TFTs in which polysilicon film is used as elements for switching pixels or as elements that form part of the driver circuit for controlling the pixels.
Generally, as the method for obtaining the polysilicon film, amorphous silicon film is crystallized to provide polysilicon film. Recently, attention has been given particularly to a method of crystallizing amorphous silicon film by using laser light. In this case, crystallization by laser makes it possible to achieve the crystallization by heating only the semiconductor film such as the amorphous silicon film. Therefore, this method is an effective method for forming a crystallized semiconductor film on a substrate of low heat resistance such as glass substrates or plastic substrates. This crystallization method for semiconductor film by using laser light is disclosed in detail in JP 2001-44120 A (hereinafter, referred to as patent document 1), JP H11-307450 (patent document 2) and JP 2000-505241 A (patent document 3).
The patent document 1 describes a laser heat treatment device that improves the crystallinity by using a plurality of light sources. The document further describes, in particular, the use of an ultraviolet ray as a main light source and a pulsed beam emitted from a solid state laser as a subordinate light source.
The patent document 2 discloses a thin film reformer for performing crystallization with radiation of two types of laser light as in the laser heat treatment device of the patent document 1. The document 2 further describes, in particular, that laser light which does not show large absorption for semiconductor film but shows large absorption for the substrate, such as carbonic acid gas laser, is used to improve the crystallinity of the substrate, which makes it possible to improve the characteristics of transistors or the like fabricated on the substrate of improved crystallinity.
The patent document 3 discloses a crystallization process of a semiconductor region on the substrate wherein a linear or slit-shaped beam is applied to a semiconductor region on the substrate in the lateral direction so as to make such crystal growth that crystals of the semiconductor film are grown largely in the lateral direction. However, in the case of the crystallization process for the semiconductor region on the substrate disclosed in the patent document 3, the distance of lateral growth by one time irradiation is about 1 micron to 2 microns. Therefore, the above-described crystal growth needs to be repeated as required in order to crystallize a large-area semiconductor film.
Various laser devices are available for laser devices to be used for the methods described above. First of all, in terms of the form of oscillation, laser devices are roughly divided into pulsed laser devices that perform pulsed oscillation and continuous wave laser devices that perform continuous oscillation. Although both devices are used for crystallization of the semiconductor film, yet the pulsed laser devices are widely used for crystallization of semiconductor film because of an advantage capable of instantaneously giving large power.
Currently, pulsed oscillation type excimer laser devices are available as laser devices which are commonly used for crystallization of the semiconductor film. In the devices, the repetition frequency of pulsed oscillation is about 1 Hz to 300 Hz. The excimer laser device is large in output power, and the oscillation light is high in absorption coefficient for silicon film because of the oscillation light being ultraviolet rays. Moreover, the oscillation light of the excimer laser device is capable of instantaneously heating by virtue of their short pulse width. Thus, the excimer laser device has an advantage that making the semiconductor film fused does not involve so much increase in the substrate temperature.
However, excimer laser devices need such gas as krF (wavelength: 248 nm) or XeCl (wavelength: 308 nm) for oscillation, and gas supply units for these gases are expensive. Further, since replacement of gas, replacement of oscillating tubes, replacement of optical windows and the like are regularly necessitated. This disadvantageously leads to high maintenance cost.
Further, other laser devices with the medium given by argon gas or carbonic acid gas have also been used as gas laser devices. In particular, carbonic acid gas laser devices are high in efficiency, allowing high output power to be obtained with relatively small-sized equipment.
Besides, it is also possible to use laser light derived from an oscillation source given by a solid state laser device (a laser device that outputs laser light with a crystal rod used as its resonant cavity). Such solid state laser devices are given by commonly known ones, being exemplified by YAG lasers (which normally mean Nd:YAG lasers), Nd:YVO4 lasers, Nd:YAlO3 lasers, ruby lasers, Ti:sapphire lasers, glass lasers and the like. Since YAG lasers have a fundamental wave (first higher harmonic) whose wavelength is as long as 1064 nm, the second harmonic (wavelength: 532 nm), the third harmonic (wavelength: 355 nm) or the fourth harmonic (wavelength: 266 nm) is used in some cases. It is noted that the fundamental wave can be modified to the second harmonic, the third harmonic or the fourth harmonic by a wavelength modulator including nonlinear elements. The formation of those harmonics is performed according to known techniques.
Also, in some cases, the Q-switching method (Q-modulation switching method) is used. The Q-switching method is often used for the YAG lasers. This is a method that the Q value is abruptly increased from a sufficiently low Q value state of the laser resonator to produce a sharp pulsed laser of quite high an energy value. In this method, the repetition frequency of the pulsed oscillation is 100 to several tens kHz. These are known techniques.
Although various semiconductor film crystallization methods using laser devices have been proposed as described above, there have been also proposed methods for crystallizing a semiconductor film in combination with the plural kinds of laser devices as well as a single laser device.
Further, in the prior art, there have been provided many proposals for the method of crystallinity improvement and the method of throughput (processing speed per unit time) improvement in the process of performing the crystallization of semiconductor film by using laser light.
However, the prior arts described in the aforementioned patent documents 1 to 3 have the follow disadvantages.
Both the patent documents 1 and 2 describe that the crystallinity of the semiconductor film can be improved with the use of two types of laser light. In particular, the patent document 1 discloses in detail the relations among the irradiation intensity of laser light, the mobility of transistors and the size of crystal grains. However, as an example, the mobility is 100 cm2/Vs to 150 cm2/Vs at most, the value being very low as compared with single crystal silicon or the like.
This is because only using plural types of laser light does not allow acceleration of the crystal growth so much, and therefore, to achieve a drastic increase of the size of crystal grains, as stated in the patent document 1. As a consequence, it is impossible to improve the characteristics of transistors by improved crystallinity of the semiconductor film.
In the patent document 3, laterally elongated crystals are made, thereby, transistors are formed with the channel direction coincident with the growth direction of the crystals, which makes it possible to fabricate transistors having a mobility of 300 cm2/Vs to 400 cm2/Vs or over. However, the length of crystals grown by one-time irradiation is 1 micron to 2 microns as described above, and therefore, there is a need for stringing the crystals one after another, which leaves a great issue unsolved in terms of throughput.
Furthermore, in the prior arts disclosed in the patent documents 1 to 3, there occurs a protrusion, so called “ridge”, at each grain boundary portion of the formed crystals. The ridge is caused by collisions of crystals that have grown in different directions. The ridge has a height comparable to film thickness of the semiconductor film to be crystallized. Then, if the channel portion of a transistor for example is formed at a portion where the ridge is generated, there occurs a phenomenon that electric fields concentrate to the ridge (protrusion) portion to incur a breakdown, which gives rise to an issue of deterioration in reliability of the transistors. Besides, the ridge portion is thick in film thickness and has defects concentrated thereabout, transistors whose channel is formed at the ridge portion are generally poor in characteristics and not for practicable use.
In addition, the patent document 3 discloses a method where a transistor is formed in no ridge region which is formed between lateral crystals after stringing the lateral crystals one after another. Though it is possible to form a transistor in no ridge region, deterioration of throughput is inevitable in ensuring large areas free from ridges. Besides this, transistors need to be placed so as not to overlap the ridges. In this case, the transistor is placed after the completion of crystallization and forming a pattern. Therefore, it is necessary to predict the ridge positions preparatorily with an extremely high accuracy and place the ridges so that interference with the ridges does not occur in the later process of forming the transistors. The placement accuracy of the ridges in such a case needs to be, generally, equivalent to that of the placement of the transistors. For this purpose, equipment for performing the crystallization process in semiconductor regions on the substrate generally requires a level of accuracy equivalent to that of exposers for pattern formation, thus the price of the equipment being very expensive.
In the method of forming the lateral crystal growth in the semiconductor regions on the substrate disclosed in the patent document 3, fabrication of flat crystals free from ridges is hard to achieve under practical conditions although achievable under certain limited conditions.
In order to manufacture high-performance thin film transistors, it is necessary to obtain crystals having flat surfaces, less defects and large crystal grains at high throughput. However, it is hard to make the crystals that satisfy these characteristics at the same time in the prior arts disclosed in the patent documents 1 to 3. That is, from the patent documents 1 to 3, it is impossible to obtain a practical crystallization method which satisfies both the obtainment of crystals having high-performance crystallinity: less defects and less grain boundaries and flat surfaces, and the capability of high throughput at the same time even if any types of laser light either alone or in combination is used. In other words, the prior arts disclosed in the patent documents 1 to 3 make it possible only to obtain crystals having a capability of fabricating thin film transistors for liquid crystal panel used at a level of 100 cm2/Vs to 200 cm2/Vs in mobility.